The present invention is directed to rotating electric machines used as traction for a hybrid electric vehicle (HEV) or electric vehicle (EV).
Rotating electric machines are mounted in household electrical appliances and office automation machines, and in recent years, they have also come to be mounted on HEVs, EVs, and other electrically driven vehicles.
Rotating electric machines for electrically driven vehicles such as HEVs or EVs, in particular, are required to develop high motive output power. These rotating electric machines for electrically driven vehicles are used in wide rotational speed ranges, so in each of the rotating electric machines, an excitation frequency of an electromagnetic excitation force varies in a wide range and a natural frequency of a structure of the rotating electric machine agrees with the excitation frequency at specific rotational speeds. The occurrence of vibration/noise due to resonance is therefore unavoidable.
The pursuit of a more comfortable vehicle interior environment, on the other hand, is increasing the need for reduction in vibration/noise level, and a number of techniques are developed to reduce the vibration/noise arising from the rotating electric machine main body.
The electromagnetic excitation force that causes the vibration/noise arising from the rotating electric machine main body is applied in three directions, that is, radially, tangentially, and axially. Reducing noise of an audible frequency band, in particular, requires reducing harmonics amplitude of these electromagnetic excitation forces.
Patent Document 1 discloses a rotating electric machine having an “n” number of poles and an “s” number of phases. In-phase teeth each with a coil of the same phase placed in each of two slots, and out-of-phase teeth each with a coil of a different phase placed in each of two slots formed along a rotational axis of a rotor are arranged in a stator core of the rotating electric machine. In this rotating electric machine, when an alternating current is supplied to the “n×s” number of coils, a magnetic circuit is formed across each of the in-phase teeth and the out-of-phase teeth, with a difference being provided in magnetic resistance of each magnetic circuit so that the number of magnetic fluxes passing through the teeth will be substantially equal between the two sets of teeth.
The rotating electric machine described in Patent Document 1 is of Intelligent Power Module (IPM) construction with a rotor of “n” poles, wherein the number of stator core teeth is limited to “2n×s”. It is described that such a shape of the stator core reduces an electromagnetic excitation force of 6f (sixth order) and thus reduces electromagnetic noise. In the stator core structure of this rotating electric machine, the number of slots is limited to “2n×s” (“2n” poles by “s” phases), so the rotating electric machine needs further improvement or modification to achieve application to other forms of stator core structures in terms of the number of slots.